This invention relates to liquid metal cooled fast breeder nuclear reactors.
In a liquid metal-cooled fast breeder nuclear reactor, the fuel assembly comprises a multiplicity of slender fuel elements or pins over which liquid metal coolant such as sodium is flowed. For convenience, the fuel assembly is divided into a plurality of interchangeable sub-assemblies, each comprising a plurality of fuel pins enclosed by a wrapper and having a lifting head. The sub-assemblies are positioned in side-by-side array and each one is located in cantilever manner by a lower end spike which is plugged into a fuel assembly supporting diagrid. The sub-assemblies in the central region of the fuel assembly mainly comprise fissile material whilst the sub-assemblies in the surrounding outer region comprise breeder material. During operation of the nuclear reactor the breeder material captures neutrons emitted by fission in the fissile material to produce further fissile material. As irradiation of the fuel assembly progresses and the fissile content of the breeder material increases, fission of some of the newly formed fissile material takes place so that the power output and, therefore, the coolant temperature from the breeder sub-assemblies progressively increases. Increased and varying temperatures of coolant streams flowing from the fuel assembly give rise to a condition known in the fast reactor art as thermal striping, a condition which, because of rapid temperature fluctuations, causes cracking in reactor structure material. In co-pending application Ser. No. 964,350 by Edward Duncombe and Gordon Thatcher there is disclosed a liquid metal cooled fast breeder nuclear reactor wherein each of at least some of the breeder sub-assemblies has an electromagnetic brake for regulating the flow of coolant through the sub-assembly, the magnetic fields of the electromagnetic brakes being temperature sensitive. In an electromagnetic braking device a liquid metal flow duct is directed through a transverse magnetic field so that when liquid metal flows through the duct and magnetic flux a retarding force in opposition to the fluid flow is set up by electromagnetic interaction. The electromagnetic brake serves to restrict flow of coolant through the breeder sub-assemblies the restriction being temperature dependent and reducing with increased temperature so that, with increased power output, coolant flow is increased thereby maintaining the outlet temperature of the coolant substantially constant. The reduced restriction is caused by reduced magnetic flux permeability of the magnetic materials caused by increased temperature. Two disadvantages are envisaged with the construction described in application Ser. No. 964,350 firstly, when the restriction to flow due to the electromagnetic brake is high, coolant pressure in the sub-assembly wrapper is high causing dilation of the wrapper under irradiation creep, and secondly, an electromagnetic brake of sufficient capacity is of relatively large proportions and therefore difficult to accommodate in the fuel sub-assembly structure.
An object of the present invention is to provide a liquid metal-cooled fast breeder nuclear reactor having breeder fuel sub-assemblies with electromagnetic brake means of relatively small proportions and which seeks to maintain the outlet coolant temperature of at least some of the breeder sub-assemblies substantially constant throughout the life of the fuel assembly without severely pressurising the sub-assembly.